The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In the field of an automatic transmission, more multiplicity of shift-stages is useful technology for enhancement of fuel consumption and drivability of a vehicle.
Achieving more shift stages contributes to improving performance and efficiency of a vehicle in a so-called engine downsizing trend, and better linearity of the shift-stages enhances drivability of the vehicle by improving uniformity of acceleration before and after a shifting, etc.
In order to achieve more shift-stages for an automatic transmission, the number of parts is typically increased, which may deteriorate installability, production cost, weight and/or power flow efficiency.
In order to enhance fuel consumption of an automatic transmission having more shift-stages, an eight-speed automatic transmission has been introduced recently, and a planetary gear train for an automatic transmission enabling more shift-stages has been developed.
A recent eight-speed automatic transmission typically shows a gear ratio span, which affects the linearity of step ratios, in a level of 6.5 to 7.5.
In addition, an eight-speed automatic transmission may become inferior in respect of linearity of step ratios of shift-stages even if the gear ratio span may become a level above 9.0. Thus, high efficiency automatic transmission providing at least nine forward speeds may be more appropriate for enhancing driving efficiency and drivability of a vehicle.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the present disclosure and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art.